256 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



in the various classes of vertebrates. As stated above the lungs arise as 

 a diverticulum (fig. 264, A) on the ventral side of the pharynx which 

 quickly divides into two sacs, the anlagen of the two lungs. These are 

 gradually pushed posteriorly toward the body cavity, still retaining 

 their connexion with the pharynx by the air duct, and each consisting 

 of an enlarged terminal vesicle connected by a slender portion (the 

 beginning of the primary bronchus) with the undivided tracheal portion. 

 With continued growth each terminal vesicle divides again and again, 

 the result being a number of rounded vesicles connected with the pri- 

 mary bronchi by slender tubes, the secondary bronchi (fig. 264, B). 



FIG. 264. FIG. 265. 



FIG. 264. Two stages in the development of the lung of the pig, ventral views, after 

 Flint. A, pig 5 mm. long; B, 18.5 mm. long, b, gill pouch; d, I, v, dorsal, lateral and 

 ventral bronchi; oe, oesophagus; t, trachea. 



FIG. 265. Scheme of mammalian lung structure, ad, alveolar duct; b, bronchus; 

 //, bronchiole; i, infundibulum lined with alveoli. 



By a continuation of this process tertiary and other bronchi are out- 

 lined, and also slender tubes, the bronchioles, to be described later, 

 which connect the terminal vesicles with the ultimate bronchi. Next, 

 the inner wall of each vesicle becomes divided into small chambers, the 

 alveoli, the whole vesicle now being known as an infundibulum. 

 The result of these many divisions is an enormous amount of internal 

 respiratory surface without great increase in the size of the whole 

 organ. It is to be noticed that in this subdivision the entodermal li- 

 ning takes the initiative, the outer (serous) surface showing but slight 

 signs of the internal modifications. 



Each infundibulum has its own duct which, when smooth internally, 

 is called a bronchiole, when lined with alveoli, an alveolar duct. 



